The present invention relates generally to a branching unit for underwater cables. More particularly, the invention relates to a branching unit for underwater telecommunication cables having a circuit in series with sea ground that limits peak current flowing through a relay in the branching unit, which extends the life of the relay and prevents any arc inside the relay envelope due to high current when the relay contact closes that could cause a main trunk cable to be shorted to sea earth.
Underwater or submarine cable systems were originally designed to provide a telecommunications link between two landing points separated by a body of water, such as France and England. Optical fiber within the submarine cable could carry high bandwidth telecommunications across tens of kilometers without the need for amplification or regeneration.
As applications evolved that required transmission across larger distances, optical repeaters in the form of regenerators or amplifiers were required within the cable span, which mandated the availability of electrical power. Power feed lines and optical transmission lines were provided together in the submarine cables. These cable systems were soon improved to provide a telecommunications connection to three or more separate landing points by employing a branching unit at the junction of multiple cables under the body of water. A standard branching unit connected a main cable from one landing point and two spur cables from second and third landing points, respectively, in a Y-shaped arrangement.
FIG. 1 illustrates a typical three landing point connection scheme. Branching unit 100 physically interconnects the cables, coordinates the routing of the cables, and provides for power switching between the cables, among other things. Conventionally, stations at the landing points power repeaters 120 within the three cables in a two-sided and one-sided arrangement. In particular, the transmission of electrical direct current along a power feed line of the intervening cables is established between any two of the landing points, and the third landing point passes electrical current through its respective cable to a ground at sea earth. Relays within the branching unit establish the two-sided (bilateral) and one-sided (unilateral) feeding configuration and help to change it if a fault arises in one of the branches. More elaborate schemes also exist for connecting multiple landing points and multiple branching units in a submarine network.
The act of configuring a branching unit refers to coordinating the application of power from the respective landing points by providing a first power connection (two-sided) between any two landing points and a second power connection (one-sided) between the third landing point and sea earth. The two-sided power connection between the above two landing points can be either a two-end feeding as shown in FIGS. 2A, 2B, i.e., a generator is present at each of the two landing points or, alternatively, a one-end feeding, i.e., a generator is present at one of the landing points and the cable is grounded at the other of the two landing points.
FIGS. 2A and 2B illustrate the two configuration steps. As shown in FIG. 2A, a low current is caused to flow from landing point A to landing point C via branching unit 100. In this arrangement, the line from A to C is nominated as the main trunk, and the spur branch to landing point B is left in an open-circuit condition. FIG. 2B shows that after the main trunk has been established, the one-sided powering of the spur takes place by shunting the spur to sea earth at the branching unit and feeding the spur from landing point B.
Various publications describe this configuration process. U.S. Pat. No. 5,196,984 discloses a branching unit which employs electrical power feeding for repeaters and multiple branching units. The branching unit terminates three line cables and a sea earth and includes three high voltage element relays, only one of which is energized at a time. When electrical power is supplied between any two line cables, the third is isolated and connected to the sea earth. Short circuit or open circuit faults in one line cable may be isolated and connected to the sea earth, while allowing powering of the remaining two line cables that are free of faults.
Similarly, U.S. Pat. No. 5,214,312 discloses a power feed line switching circuit for a submarine branching unit having first, second and third electrical paths connected in a Y-shaped connection, and first, second and third terminals connected respectively thereto. The power feed line switching circuit also has first, second and third relays each including a drive unit inserted in the first, second and third terminals and a switching unit for disconnecting the corresponding terminal and connecting the terminal to the ground. The relays control the connections between the terminals and electrical paths for establishing a one-end power feed line or a two-end power feed line to maintain power feed for repeaters and the submarine branching unit by the plurality of relays.
U.S. Pat. No. 5,644,466 discloses a submarine cable branching system including a plurality of directional relays in respective power feed paths branched at a common node. Each of the directional relays is energized by a current flowing through the power feed path in a predetermined direction for energizing a corresponding switch provided in a different power feed path to establish a bilateral feed path and further a unilateral feed path. A bypassing switch is provided across one of the switches that forms the bilateral feed path such that the bypassing switch is urged to close in response to energization of a self-sustaining relay provided in the unilateral feed path.
Since the branching unit, repeaters and cables are laid underwater and are thus difficult to maintain, it is important that they have a high reliability to ensure uninterrupted telecommunications between the landing points. During power-up of the main trunk, however, the spur cable will acquire a charge related to the voltage drop at the branching unit and the length of the spur. Switching the spur cable to sea earth (typically the sea water) can cause a rapid discharge, which may damage contacts and relays within the branching unit. In particular, a relay that causes the spur cable to contact sea earth is typically a high-voltage changeover relay. When actuation of the relay causes a make of the switch, an arc discharge will liberate a quantity of contact material. Acting as charge carriers, the liberated material may pass between the just opened contacts in the three-terminal relay. An avalanche effect could take place that causes the main cable to discharge across the contact gap with large amperage, leading to imminent failure or degradation of the relay and connection of the main cable to sea earth. In this scenario, either it could be impossible to power the system up and configure the system or an oscillation could be initiated between the main trunk and spur (see, e.g., U.S. Pat. No. 5,517,383 at columns 1 and 6).
U.S. Pat. Nos. 5,517,383 and 5,841,205 propose circuits to avoid this damage. In the xe2x80x2383 patent, an additional relay D and switch D1 are used to avoid dangerous arcing in the vacuum changeover relay such as C. Relay D is not a changeover relay and can withstand a greater amount of arcing without failing.
The xe2x80x2205 patent discloses the use of a two-stage relay to avoid damage from arcing. In this arrangement, the spur is disconnected from the main trunk when a defined current level is reached and then discharged by shorting its terminal station end (i.e., not underwater). After current in the main trunk ramps to a second level, the spur is connected to sea earth at the branching unit without arcing.
Applicants have found that these prior arrangements introduce unnecessary complexity and expense to a branching unit and its operation. A branching unit needs to be designed that avoids any arc transfer and its subsequent relay damage in any worst-case scenario, including relay degradation due to system cable faults and due to the branching unit configuration sequence.
Applicants have discovered that the above problem can be overcome with a branching unit that limits the peak current to a level that the high-voltage relay used in the branching unit can withstand without arcing for a number of cycles substantially greater than the expected number of configurations.
Applicants have discovered that surge currents in a spur of a submarine branching unit that is grounded to sea earth during configuration of the branching unit may be minimized by adding a current limiter in series with the sea ground.
Applicants have further discovered that the current limiter in series with the sea ground can advantageously be constructed from at least one inductor, possibly in parallel with at least one resistor. Applicants have further discovered that a system with such a current limiter in place reduces surge currents, both during initial configuration and during switching from faults occurring on a main trunk, thus prolonging the life of the switching contacts.
In one aspect, a branching unit for joining power feed lines of at least three submarine cables consistent with the present invention includes first, second, and third cable terminations each coupled to a power feed line of a respective submarine cable, a ground termination, a current limiter positioned in series between the ground termination and sea earth, and first, second, and third high-voltage relays. Each relay has a coil and a contact, where the coil of each high-voltage relay is positioned between two of the cable terminations respectively. Each coil has an energized state when a first threshold amount of current passes through the coil and a de-energized state when a second threshold amount of current does not pass through the coil. The contact of each high-voltage relay is positioned to connect the respective third cable termination with the ground termination when the respective coil is in an energized state and to connect the respective third cable termination with at least another of the cable terminations when the respective coil is in a de-energized state. More particularly, the current limiter is an inductor. The current limiter may include at least one resistor in parallel to the inductor.
In another aspect, an underwater optical telecommunication link according to the present invention comprises at least three submarine cables extended at least partly in a body of water and having first, second and third power feed terminations at respective landing points, each cable comprising at least an optical fiber and a power feed line electrically connected to the respective power feed termination; at least two power stations at the landing points to feed said power feed lines of the submarine cables; and a branching unit according to one of the other aspects of the invention, in said body of water, for joining said power feed lines of said submarine cable.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention.